Space division multiplexing optical coherence tomography using an integrated photonic device

ABSTRACT

Integrated photonic chips and related systems and methods suitable for space-division multiplexing optical coherence tomography scanning are disclosed. In one embodiment, the photonic chip comprises a substrate, an optical input port which receives an incident sampling beam from an external light source, a plurality of optical output ports configured to transmit a plurality of sampling beams from the chip to a sample to capture scanned images of the sample, and a plurality of interconnected and branched waveguide channels formed in the substrate. Waveguide channels in a splitter region divide the sampling beam into the plurality of sampling beams at the output ports. Terminal portions of the waveguide channels in a time delay region associated with each output port have different predetermined lengths to create an optical time delay between the sampling beams. In some embodiments, the chip further comprises an interferometer region to create interference patterns.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. national stage application under 35U.S.C. § 371 of PCT/US2018/032529 filed May 14, 2018, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 62/505,199 filedMay 12, 2017, the entireties of which are herein by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under NationalInstitutes of Health (NIH R21 EY-026380, K99/R00 EB-010071) and NationalScience Foundation (NSF DBI-1455613, IIP-1640707, and IIP-1623823). Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention generally relates to optical coherence tomography(OCT) imaging systems, and more particularly to such a systemincorporating an integrated photonic chip.

Improving imaging speed is a main driving force for the development ofoptical coherence tomography (OCT). Space-division multiplexing opticalcoherence tomography (SDM-OCT) is a recently developed parallel OCTimaging method used to achieve multi-fold speed improvement. However,the assembly of multiple fiber optics components conventionally used insuch systems may be labor-intensive and susceptible to errors whichmakes it challenging for mass-production. In addition, the numerouscomponents of an OCT system consume space and are not readily amenablefor incorporation into a compact imaging device which may be used invarious medical diagnostic settings or for other uses. Improvements inSDM-OCT systems are desired.

SUMMARY OF THE INVENTION

A wide-field high-speed SDM-OCT system using an integrated photonic chipis disclosed that can be reliably manufactured with high precision andlow per-unit cost facilitating the broad dissemination of the SDM-OCTtechnology. The present chip takes advantage of advances in the field ofsilicon photonics. In one embodiment, the present photonic chip replacesat least the fiber-based optical components of the traditional OCTsystem with on-chip photonic components to improve equipment reliabilityand permit creation of mass-producible compact SDM-OCT imaging devices.

The present chip-based SDM-OCT system can improve the imaging speed by afactor of 10 or more while maintaining the sensitivity in variousapplications, including ophthalmology, cardiovascular imaging,endoscopic imaging, cancer imaging, dental applications, researchimaging applications, and others.

In one embodiment, the photonic chip may comprise on-chip combinedsplitter and optical time delay regions or areas formed by a pluralityof interconnected branched waveguides formed in a substrate which defineoptical splitters. Multiple branches of waveguides and cascading rows ofsplitters may be provided in some configurations to successively splitan incident singular sampling beam in each row into a plurality ofsampling beams output from the chip. The waveguides may be channelsformed by etching or deposition such as doping the substrate whichcreate differential indices of refraction that force the light signalsor waves to travel within the waveguides. In some embodiments, thesubstrate may be formed of solely silicon or a composite silicon oninsulator (SOI).

The chip may include a single optical input port which receives asampling light beam generated by a long coherence light source andplurality of spatially separated optical output ports for scanningmultiple sampling beams simultaneously in parallel onto the surface of asample to be scanned by the SDM-OCT system. Terminal portions of thewaveguide channels associated with each output port have predetermineddifferent lengths to create an optical time delay between each samplingbeam for capturing space division multiplexed OCT scanned images of thesample. Sampling light signals reflected from the sample are returnedsimultaneously in parallel back through the same on-chip splitter andtime delay structure following the sampling light path in reversedirection for further processing as described herein to generate digitalimages of the sample using one or more interferometers and additionalimage processing devices described herein.

In another embodiment, a low insertion loss photonic chip is disclosedherein which further includes an on-chip photonic interferometer unit,different sampling and reflected light signal paths through the chip,and a reference light input port. The various waveguide channels areconfigured and operable to process sampling light signals, reflectedlight signals, and reference light signals to create interferencesignals on the chip. A plurality of interference signals are emittedfrom the chip simultaneously in parallel which are further processed tocreate digital scanned images of the sample as further described herein.

In one aspect, an integrated photonic chip suitable for space-divisionmultiplexing optical coherence tomography scanning comprises: asubstrate; an optical input port configured to receive an incidentsingular sampling beam from an external light source; a plurality ofoptical output ports configured to transmit a plurality of samplingbeams from the chip to a sample to capture scanned images of the sample;and a multiple branched waveguide structure optically coupling the inputport to each of the output ports, the waveguide structure comprising aplurality of interconnected waveguide channels formed in the substrate;the waveguide channels configured to define a plurality of photonicsplitters which divide the incident singular sampling beam received atthe input port into the plurality of sampling beams at the output ports;wherein portions of the waveguide channels between the photonicsplitters and output ports have different predetermined lengths tocreate an optical time delay between each of the plurality of samplingbeams. The light source may be a wavelength-tunable long coherence lightsource in one embodiment. A difference in the predetermined lengthsbetween the waveguide channels is selected to produce an optical delayshorter than a coherence length of the light source between theplurality of sampling beams so that when images are formed, signals fromdifferent physical locations are detected in different frequency bands.

In another aspect, a low loss integrated photonic chip suitable forspace-division multiplexing optical coherence tomography scanningcomprises: a substrate; an optical input port configured to receive anincident singular sampling beam from an external light source; areference light input port configured to receive reference light from anexternal reference light source; a plurality of optical output portsconfigured to transmit a plurality of sampling beams from the chip to asample to capture scanned images of the sample; a multiple branchedwaveguide structure optically coupling the input port to each of theoutput ports, the waveguide structure comprising a plurality ofinterconnected waveguide channels formed in the substrate, the waveguidechannels defining a splitter region and an interferometer region; thewaveguide channels in the splitter region configured to define aplurality of photonic splitters which divide the incident singularsampling beam received at the input port into the plurality of samplingbeams at the output ports; wherein portions of the waveguide channelsbetween the photonic splitters and output ports have differentpredetermined lengths to create an optical time delay between each ofthe plurality of sampling beams; the waveguide channels in theinterferometer region configured to define a plurality of photonicinterferometers, the photonic interferometers optically coupled to thewaveguide channels in the time delay region and the reference light;wherein the photonic interferometers are arranged to receive a pluralityof reflected light signals returned from the sample, the photonicinterferometers being configured and operable to combine the reflectedlight signals with the reference light to produce a plurality ofinterference signals which are emitted from interference signal outputports of the photonic chip. The light source may be a wavelength-tunablelong coherence light source in one embodiment. A difference in thepredetermined lengths between the waveguide channels is selected toproduce an optical delay shorter than a coherence length of the lightsource between the plurality of sampling beams so that when images areformed, signals from different physical locations are detected indifferent frequency bands.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the exemplary embodiments will be described withreference to the following drawings where like elements are labeledsimilarly, and in which:

FIG. 1 is a schematic diagram of a space-division multiplexing opticalcoherence tomography (SDM-OCT) system incorporating a photonic chipaccording to one embodiment of the present disclosure;

FIG. 2 is a schematic diagram of the photonic chip of FIG. 1 showing anexemplary waveguide architecture;

FIG. 3 is a photographic image of a prototype chip of FIG. 1 for sizecomparison to U.S. currency;

FIG. 4 is a diagram or chart showing roll-off measurement of the centralbeam of the chip-based SDM-OCT in logarithmic scale;

FIG. 5 is a 1951 USAF resolution test target image showing transverseresolution of the chip-based SDM-OCT prototype system;

FIG. 6 is a schematic diagram of a space-division multiplexing opticalcoherence tomography (SDM-OCT) system incorporating a low insertion lossphotonic chip according to another embodiment of the present disclosure;

FIG. 7 is a schematic diagram of the photonic chip of FIG. 6 showing anexemplary waveguide architecture;

FIG. 8 is the schematic diagram of FIG. 6 showing the standard non-chipoptical components and devices of the system which can be replaced bythe on-chip waveguide photonic devices created by the photonic chip;

FIG. 9 is a schematic diagram of a first alternative embodiment of thechip of FIG. 7 with directly attached on-chip photodetectors; and

FIG. 10 is schematic diagram of a second alternative embodiment of thechip of FIG. 7 with directly embedded in-chip photodetectors formedintegrally with the chip.

All drawing shown herein are schematic and not to scale. Parts given areference number in one figure may be considered to be the same partswhere they appear in other figures without a reference number forbrevity unless specifically labeled with a different part number anddescribed herein.

DETAILED DESCRIPTION

The features and benefits of the invention are illustrated and describedherein by reference to non-limiting exemplary embodiments. Thisdescription of the embodiments is intended to be read in connection withthe accompanying drawings, which are to be considered part of the entirewritten description. Accordingly, the invention expressly should not belimited to such embodiments illustrating some possible non-limitingcombination of features that may exist alone or in other combinations offeatures; the scope of the invention being defined by the claimsappended hereto.

In the description of embodiments disclosed herein, any reference todirection or orientation is merely intended for convenience ofdescription and is not intended in any way to limit the scope of thepresent invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivative thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures may be secured or attached to oneanother either directly or indirectly through intervening structures, aswell as both movable or rigid attachments or relationships, unlessexpressly described otherwise.

Recently, in commonly owned U.S. Pat. No. 9,400,169 incorporated hereinby reference in its entirety, a parallel OCT imaging method, namelyspace-division multiplexing OCT (SDM-OCT), was disclosed which is basedon a single source and detection unit. In SDM-OCT, the sample arm beamis split into multiple channels, with different optical delays in eachchannel in order to generate multiplexed interference signals that canbe detected simultaneously. The optical delays were created in oneembodiment with optical fiber-based elements comprising a planar lightwave circuit splitter and an optical delay element comprising aplurality of optical fibers each having a different length to producedifferent optical delays in the sampling light beams transmitted to andreflected back from the sample (see, e.g. FIG. 1 in U.S. Pat. No.9,400,169). Split beams were used to image different segments of thesample, hence the imaging speed was increased by a factor equal to thenumber of beams. A high imaging speed of 800,000 A-scans/s wasdemonstrated in this first prototype system by creating eight-beamillumination using a long-coherence-length VCSEL laser running at 100kHz.

Although the foregoing SDM-OCT offered scalable speed improvement with asimple system configuration, the inventors discovered that the first labprototype SDM-OCT required extensive time and manual effort to assemblethe multiple custom fiber components and control optical delays for eachchannel, which limits the broad dissemination of the SDM-OCT technology.

SDM-OCT System With Prototype Integrated Photonic Chip

To construct SDM-OCT easily and reliably, the present disclosureeliminates the foregoing optical fiber-based delay elements which arereplaced using silicon photonics. The present invention provides anSDM-OCT system 100 with an integrated photonic chip 101 comprisingphotonic components configured to produce optical delays in the samplingand return path light beams. Integration of such components onto aphotonic chip has advantages of cost, size and stability of the system.The progress in nano/micro-fabrication of these photonic integratedcircuits (PICs) has leveraged on the significant advancement in theSi-based processing capabilities, which in turn results in significantcost reduction in large scale manufacturability of such technology. ThePICs also provide the ability to achieve new functionalities withincreased yield and reduced errors in packaging. Integration of OCTcomponents such as interferometer and spectrometer onto a photonic chiphas been previously reported. In the case of SDM-OCT system, using a PICchip to replace fiber-based space-division multiplexing components ispotentially advantageous, since custom optical delays and spacingbetween each output beam can be precisely defined lithographically withsub-micron tolerances during the fabrication process.

FIG. 1 shows a schematic diagram of a prototype photonic chip-basedSDM-OCT system 100 used in the test setup to demonstrate the performanceof such a system. In this prototype, traditional fiber-based componentsused to produce the optical delay between the sampling beam channels orpaths, including a planar light wave circuit splitter and a fiber arraywith different optical delays, were replaced by a single integratedphotonic chip 101. The schematic layout of the silicon-based photonicchip 101 is shown in FIG. 2. FIG. 3 depicts an actual photographic imageof the fabricated prototype chip. Photonic chip 101 comprises asubstrate 102 which may have a generally rectangular prismatic or cuboidconfiguration in one embodiment including two opposing parallel majorsurfaces 101 a, 101 b defining a thickness T measured therebetween andfour perpendicular side surfaces 101 c defining a perimeter of the chip.Substrate 102 is formed of a material having a first refractive indexRI-1. In the prototype system, the substrate 102 had a thickness T1 ofabout 1-2 mm. Other thicknesses, however, may be used for substrate 102.

The substrate 102 of photonic chip 101 may be made of any suitablesingle material or multi-layered composite combination of materialsconventionally used for constructing a photonic chip with waveguides,such as for example without limitation silicon or silicon-on-insulator(SOI). In one embodiment, photonic chip 101 is constructed of an SOIsubstrate. SOI chips typically comprise a silicon (Si) base layer,intermediate silicon dioxide (SiO₂) insulator layer, and thin topcrystalline silicon layer typically with a thickness less than theinsulator layer. The top silicon layer which guides the light beams orwaves has a refractive index n=3.45 and the SiO₂ insulator layer has arefractive index n=1.45. Other materials besides silicon, such as IndiumPhosphide (InP), Lithium Niobate (LiNbO₃), Silicon Nitride (Si₃N₄) andGallium Arsenide (GaAs), etc., however may be used in other embodiments.

With continuing reference to FIG. 2, the photonic chip 101 is patternedwith a waveguide structure having an array or plurality ofinterconnected branched waveguides. The waveguides may be in the form ofwaveguide channels 103 in the illustrated embodiment configured tocreate on-chip photonic beam splitter and optical time delay units orregions. Waveguide channels 103 direct and guide the incident beam onchip 101 to propagate and follow the optical light paths indicated inthe figure through the chip, thereby advantageously allowing channels ofdifferent lengths to be created in the time delay region which producean optical delay between the channels for a space division multiplexingOCT system.

The patterned waveguide channels 103 may be formed in substrate 102 byconventional semiconductor fabrication techniques or methods known inthe art. An example of a suitable semiconductor method that may be usedis a combination of photolithography or deep UV (ultraviolet)lithography to define the desired waveguide channel pattern followed byselectively etching the Si top layer in the case of an SOI chip 101 toform the waveguides. The comparatively large difference in therefractive indices noted above between the SiO₂ insulator layer (n=1.45)and Si top layer (n=3.45) noted above confines the electromagnetic fieldinto the top Si layer causing the electromagnetic light signals or wavesin the optical spectrum to travel within the confines of waveguidechannels 103 in the photonic chip 101.

Another example method that may be used for forming waveguide channels103 is doping the substrate in the manner well known and used in thefabrication of semiconductors. Doping may involve processes such asdiffusion or ion implantation to introduce a dopant element to selectareas of the silicon substrate to create the desired pattern ofwaveguide channels 103. The doped channels have a refractive index RI-2different than the base silicon material refractive index RI-1, therebycausing the light signals or wave to follow the doped channel pattern.Other semiconductor fabrication techniques beyond those noted above usedin silicon photonics however may be used in other embodiments.

In the prototype chip-based SDM-OCT system 100, the amplified incidentbeam from light source 110 was directly coupled and input into the chip101 with an optical fiber 104 from the circulator 140 as shown inFIG. 1. Chip 101 includes an input port 106 formed on a first one of theside surfaces 101 c which directly couples to input optical fiber 104and a plurality of output ports 105 formed on a different second one ofthe side surface 101 c. Of course in other embodiments, the input andoutput ports 106, 105 may be formed on any two different side surfaces101 c of the photonic chip 101 depending on the locations of the theseports desired for the scanning device. The sides selected for the inputand output ports 106, 105 may vary and are dependent upon efficient useof chip space to minimize the size of the chip and/or to optimize thearrangement for the physical instrument or equipment in which the chipwill be integrated. Accordingly, the arrangement does not limit theinvention and the illustrate embodiment represents one of many possibleconfigurations possible.

The prototype photonic chip 101 had the following design parameters:Wavelength range: 1310+/−60 nm; Coupling: match SMF-28 fiber mode (9.2um MFD) at input and output; Output pitch p=250 um (0.25 mm); Waveguidepath length difference ΔL=2.5 mm; Index contrast: 1.5%; Waveguidedimensions: 4.0 um (Width)×4.5 um (Height); and Bending radius: 2.3 mm.Other design parameters may be used for other embodiments andapplications.

The photonic chip 101 comprises an on-chip splitter and time delayformed by specially configuring the multiple branched waveguidestructure created using the waveguide channels 103. In the prototype, asan example without limitation, a horizontal three-row cascade of 1×2photonic waveguide splitters 107 formed by multiple branched on-chipwaveguide channels 103 were used to evenly and gradually split theincident sampling light S1 in each row from the initial singular beam orchannel into final 8 beams or channels (dashed lines in FIG. 2indicating each row of splitters on the chip). Each waveguide splitter107 is formed by a branch in the waveguide which divides the inputsampling beam S1 equally (i.e. 50/50) into two output sampling lightS1beams. This occurs successively in each of the 3 rows of waveguidesplitters for convenience to create the 8 output sampling beams S1 inthe illustrated prototype embodiment developed. However, in otherembodiments a lesser or greater number of rows including even a singlesplitter row (e.g. 1×8 splitter in this example) may be used to splitthe sampling light S1 into the desired number of sampling beams forscanning the sample. The number of rows of splitters used does not limitthe invention and may be dictated in some embodiments by the geometryand/or size of the photonic chip 101 desired for the given application.It further bears noting that more or less than eight sampling beams orchannels may be used in other embodiments as needed and the invention isexpressly not limited to the eight beam prototype embodiment.

Each of the 8 beams of sampling light S1 were then transmitted in aseparate waveguide channel 103 through the chip, thereby forming aplurality of output beams or channels emitted from photonic chip 101through a plurality of output ports 105 clustered together on one side101 c of substrate 102, as shown in FIG. 2. This figure includes azoom-in view of output ports 105 of the chip showing eight waveguideoutput ports 105 forming output beam channels with a pitch p=0.25 mm (or250 μm) spacing between them. Optical delays between each of the 8waveguide channels 103 were created in the photonic chip 101 by settingdifferent terminal path or channel lengths for each channel between thethird row photonic splitters 107 and output ports 105, with a physicallength (optical delay) difference ΔL in this non-limiting embodiment ofabout 2.5 mm in the waveguide or about 3.7 mm optical delay in airbetween adjacent channels, in order to generate multiplexed interferencesignals. The time or optical delay was created by varying the lengths ofeach waveguide channel 103 between the third row of the waveguidesplitters 107 and the output ports 105 as shown. In a preferredembodiment, the difference ΔL is selected to produce an optical delayshorter than the coherence length of the light source between theplurality of sampling beams so that when images are formed, signals fromdifferent physical locations are detected in different frequency bands.

In one embodiment, a uniform or equal difference in length ΔL betweeneach adjacent waveguide channel 103 may be provided for transmittingsampling light of all wavelengths in different bands. However, the delaydoes not need to be uniform. For some applications, as an example, thesystem designer may intentionally use non-uniform delays to accommodatethe specific sample geometry to be scanned for example where the samplehas a non-uniform and/or non-planar surface geometry in order tooptimize the scanned images returned from the sample. The invention istherefore not limited to a uniform difference in length ΔL between eachadjacent waveguide channel 103.

The outputs ports 105 of the waveguide photonic chip 101 were 8-degreeangle polished to reduce back reflections. The overall size of theintegrated photonic chip was about 2.5×2.0 cm², close to the size of aU.S. quarter coin (see, e.g. photo image of FIG. 3).

In the prototype arrangement of photonic chip 101 shown in FIG. 2, thethree-row cascading splitters are arranged to split and guide thesampling light S1 beam in a first direction (downward in this figure).The terminal portions of the waveguide channels 103 associated with eachoutput port in the time delay region of the chip, which have differentpredetermined lengths to create an optical time delay between each ofthe plurality of sampling beams or channels, are arranged generallyperpendicularly to the waveguide channels 103 in the foregoing splitterregion. Thus at least in the prototype chip, the incident sampling lightS1 following the waveguide channel path in the time delay region travelsand progresses generally perpendicularly to the sampling light path inthe splitter region which advantageously conserves space on the chip 101to minimize its size, thereby allowing creation of an extremely smallphotonic splitter and time delay unit. The term “generally” is used toconnote that the sampling light S1 in the splitter region does notnecessarily travel perfectly perpendicular to the sampling light in thetime delay region when propagating through the curved and angledportions of the individual photonic splitters 107, but rather thegeneral flow of the sampling light through these regions isperpendicular to each other in this non-limiting embodiment. In otherembodiments, the flow of sampling light may be obliquely angled orparallel relative to each other in the splitter and time delay regions.Accordingly, the invention is not limited to the flow of sampling lightthrough chip 101 illustrated in the embodiment of FIG. 2.

It will be appreciated that in other embodiments besides the foregoingprototype, different numbers of waveguide channels, length or delaydifferences between channels, output spacing, polish angles, chipdimensions, and configurations of waveguides may be used. Accordingly,the invention is expressly not limited to the above design and recitedvalues of these parameters in the prototype demonstration system. Otherembodiments may therefore be different in these aspects and is notlimiting of the invention.

The rest of the prototype chip-based SDM-OCT system 100 components aredescribed as follows with additional reference to FIG. 1. The lightsource 110 may be a commercially-available wavelength-tunable, longcoherence light source to provide optimal imaging depth range. In oneembodiment, without limitation, the coherence length may be greater than5 mm to achieve proper imaging range for the SDM-OCT system. The longcoherence light source 110 used in the prototype system was a VCSEL(vertical-cavity surface-emitting laser) wavelength-tunable laser diode(e.g. SL1310V1, Thorlabs Inc., USA, coherence length >50 mm). In otherembodiments, other coherence lengths greater than 5 mm may be used. Abooster optical amplifier (BOA) 111 (e.g. BOA 1130s, Thorlabs Inc., USA)was employed in the optical path right after the commercial VCSELwavelength-tunable laser light source (e.g. SL1310V1, Thorlabs Inc.,USA) to boost the laser output from about 27 mW to about 100 mW. Thelong-coherence light source 110 was optically coupled to the BOA 111 viaan optical fiber 104. It bears noting that optical fibers 104 may beused to optically couple the remaining optical components and devices ofSDM-OCT system 100 together as shown in FIG. 1 unless describedotherwise below. Commercially available optical fibers such as Corning®SMF-28® Ultra optical fibers or others may be used to opticallyinterconnect the devices.

The amplified light emitted from the BOA 111 was passed through a 95/5optical coupler 120, with 5% of the light delivered to a customMach-Zender interferometer (MZI) 112 with an about 38.7 mm optical delayin air for phase calibration and remaining 95% of the light used for OCTimaging. The MZI light beam was first transmitted through a 50/50optical coupler 121 upstream of the MZI 112 as shown. Both outputs fromcoupler 121 are directed into the MZI which produces an interferencesignal.

The 95% of light emitted from optical coupler 120 for OCT imaging wasfurther divided by a 90/10 optical coupler 130, with 10% of the powersent to the reference arm R and 90% of the power used in the sample armS. Suitable optical couplers include optical fiber couplers availablefrom AC Photonics, Inc., Thorlabs, Inc. or other suppliers.

The incident reference arm R light beam from coupler 130 passes throughcirculator 150 and a polarization controller 151 (e.g. fiberpolarization controllers FPC commercially available from Thorlabs, Inc.USA and others). Polarization control is useful to match polarizationstate of the sample and reference arms S, R in order to optimizeinterference signals and achieve best image quality. The reference lightbeam is then transmitted through collimator 152 and lens 153 whichfocuses the reference light beam onto reference mirror 160. Thereference arm R is a non-scanning reference arm of fixed optical lengthwhich does not require changing the light path length in order to createan interference signal for coupling with the reflected light signalsfrom the sample. The reference light reflected from mirror 160 passesback through the foregoing reference arm R components to circulator 150for creating the interference signal.

The incident sampling light S1 beam from coupler 130 was coupled intothe integrated photonic chip on sample arm S through a circulator 140(e.g. AC Photonics, Inc.) and a polarization controller 154. The 8-beamsampling light S1 output from the chip (see, e.g. FIG. 2) via outputports 105 directly through air was captured and collimated by collimator170 for focusing on multiple different spots or sampling locationsacross the surface of the sample 191. A telescope setup with a first 30mm lens 171 and a second 50 mm lens 172 was used to expand the size ofthe sampling beams. A large scan lens 190 (e.g. LSM05 objective lens,Thorlabs Inc., USA) was mounted in the sample arm S after a scanningdevice such as a galvanometer having an oscillating XY galvanometermirror 173 to achieve wide-field volumetric imaging of the sample 191with SDM-OCT. The scan lens 190 had an effective focal length of 110 mmand a working distance of 93.8 mm, resulting in about 1.7 mm spacingbetween the adjacent beams at the focal plane and a transverseresolution of about 20 am as measured with a USAF target (see, e.g. FIG.5, group 4, element 5). Incident power on the sample was about 3 mW foreach beam. Laterally and spatially separated portions of the sample 191are irradiated with the sampling beams to capture digital images of thesample.

It bears noting that the sample 191 can be scanned simultaneously bysampling light S1 emitted by each of the eight output ports 105 fromphotonic chip 101 using galvanometer mirror 173. The galvanometerincludes a galvo motor with an angled vibrating/oscillating (e.g. up anddown) mirror 173 driven by a motor shaft. Sampling light beams from eachscanning output port 105 are independently transmitted and scannedacross a surface of the sample 191 by galvanometer scanner mirror 173,thereby producing discrete and independent illuminated sampling spots orlocations each corresponding to one of the output ports. Thegalvanometer mirror 173 may project the sampling beams S1 onto thesample in any suitable pattern to capture the desired image information.Other variations and types of scanning devices may be used. In somenon-limiting examples, the galvanometer scanner 200 may be CambridgeTechnologies, Model 6215H or Thorlabs, GVS102.

Reflected sample light signals S2 returned simultaneously from eachsampling location on sample 191 and reference light R1 from referencearm R from mirror 160 were routed to a 50/50 optical coupler 141 viaupstream optical circulators 140 and 150 previously described herein.During this process, reflected sample light signals travel back throughthe sample arm lenses 190, 171, and 172, collimator 170, and intophotonic chip 101. The plurality of reflected light signals S2 arecaptured by the output ports 105 and then transmitted through theplurality of waveguide channels 103 in the time delay and splitterregion in a reverse path and direction to the path and direction ofsampling light signals S1. The multiple reflected light signals arecombined into a singular reflected light signal S2 which leaves thephotonic chip 101 through the sample light input port 106 and isdirected to optical coupler 141. It bears noting the reflected lightsignal comprises a plurality of reflected light signals containing thescanned image information in different bands. Coupler 141 operates toproduce a combined reflected light interference signal comprising aplurality of interference signals based on the plurality of reflectedlight signals S2 and reference light R1.

The reflected interference signals from both the OCT via coupler 141 andinterference signal generated by MZI 112 were detected by dual balanceddetectors 180 and 181 (e.g. PDB480C-AC, 1.6 GHz, Thorlabs Inc.) andtheir outputs were acquired simultaneously by a dual-channel high-speeddata acquisition card 183 (e.g. ATS 9373, Alazar Technologies Inc.)operating at a sampling rate of 1.5 GS/s. The sampling rate wasestimated by calculating the fringe frequency based on information suchas the sweep rate and tuning range of the laser, and total imaging depthneeded by 8 parallel imaging channels. The sampling rate was kept atleast twice of the maximum fringe frequency to fulfill the Nyquistsampling requirement. Total imaging depth measured was ˜31.6 mm in airor ˜23.8 mm in tissue, which was sufficient to cover the OCT signalsfrom all the 8 beams separated with different optical delays. Thedetected signal is digitized by the high speed data acquisition card 183and streamed to an appropriately configured computer 184 to generate OCTscan images of the sample 191 captured by SDM-OCT system 100.

The acquired signal data from data acquisition card 183 is streamedcontinuously to the memory of computer 184 or memory accessible toanother suitable processor-based device or PLC (programmable logiccontroller) through a suitably configured port. The signal data may bestored on the memory for further processing, display, export, etc.

The “computer” 184 as described herein is representative of anyappropriate computer or server device with central processing unit(CPU), microprocessor, micro-controller, or computational dataprocessing device or circuit configured for executing computer programinstructions (e.g. code) and processing the acquired signal data fromdata acquisition card 183. This may include, for example withoutlimitation, desktop computers, personal computers, laptops, notebooks,tablets, and other processor-based devices having suitable processingpower and speed. Computer 184 may include all the usual appurtenancesassociated with such a device, including without limitation the properlyprogrammed processor, a memory device(s), a power supply, a video card,visual display device or screen (e.g. graphical user interface),firmware, software, user input devices (e.g., a keyboard, mouse, touchscreen, etc.), wired and/or wireless output devices, wired and/orwireless communication devices (e.g. Ethernet, Wi-Fi, Bluetooth, etc.)for transmitting captured sampling images. Accordingly, the invention isnot limited by any particular type of processor-based device.

The memory may be any suitable non-transitory computer readable mediumsuch as, without limitation, any suitable volatile or non-volatilememory including random access memory (RAM) and various types thereof,read-only memory (ROM) and various types thereof, USB flash memory, andmagnetic or optical data storage devices (e.g. internal/external harddisks, floppy discs, magnetic tape CD-ROM, DVD-ROM, optical disk, ZIP™drive, Blu-ray disk, and others), which may be written to and/or read bya processor operably connected to the medium.

It will further be appreciated that various aspects of the presentembodiment may be implemented in software, hardware, firmware, orcombinations thereof. The computer programs described herein are notlimited to any particular embodiment, and may be implemented in anoperating system, application program, foreground or background process,driver, or any combination thereof, executing on a single computer orserver processor or multiple computer or server processors

The result of a sensitivity roll-off measurement for the central beam ofthe SDM-OCT system 100 is shown in FIG. 4. The sensitivity of thechip-based SDM-OCT was measured as about 91 dB with a calibrated −47.2dB reflector as the sample. A roll-off of about 2 dB was observed overabout 27 mm depth range. The axial resolution was maintained as about 11am in air or about 8.3 μm in tissue throughout the entire imaging depth.The performance of the SDM-OCT system 100 with photonic chip 101 wassimilar to good results achieved with earlier non-chip SDM-OCT prototypesystems similar to that disclosed in FIG. 1 of U.S. Pat. No. 9,400,169.

Advantageously, the present SDM-OCT system 100 realized an eight timesincrease in imaging speed, reaching about 800,000 A-scans/s, wasachieved with the chip-based SDM-OCT system as compared to a single-spotSS-OCT with the same VCSEL running at 100 kHz. A sensitivity of about 91dB was measured with the chip-based SDM-OCT. The feasibility ofhigh-speed chip-based SDM-OCT was demonstrated with wide-field imagingcapabilities. Three-dimensional (3D) volumetric images (700×1200A-scans) of ex vivo porcine eye and in vivo human finger print coveringa large imaging area of up to 18.0×14.3 mm² was obtained in about 1second. High-definition 3D OCT images (1500×1600 A-scans) of humanfinger nail were acquired in about 3 seconds. Integrated photonicdevices can be reliably manufactured with high precisions and lowper-unit cost, facilitating the deployment and adoption of the SDM-OCTtechnology.

During the testing, two scanning protocols were employed for wide-fieldOCT volumetric imaging using SDM-OCT system 100: (1) Fast-scanning mode:OCT volumetric data, consisting a total number of 700×1200 A-scans forall eight beams (700×150 A-scans was acquired for each beam), wasobtained in about 1 second. This helps minimize motion artifacts,especially for in vivo imaging. (2) High-definition mode: OCT volumetricdata, which consisted of 1500×1600 A-scans for all eight beams (1500×200A-scans was acquired for each beam) was obtained in 3 seconds in orderto achieve Nyquest sampled transverse resolution and preserve details ofthe sample. The size of each OCT volumetric data was about 3.3 GB infast-scanning mode and ˜9.3 GB in high-definition mode. During thescanning, each beam was moved up to about 18 mm in the fast scanningaxis direction, and about 2.4 mm in the slow axis direction. Wide-fieldOCT data was obtained by stitching 3D OCT images from all 8 beams.

With respect to the foregoing optical couplers or splitters described(e.g. 120, 130, etc.), it will be appreciated that any suitable opticaldivision or splitting of input light beams identified as a percentage ofthe incident beam (e.g. 5/95, 10/90, etc.) may be used depending on theintended application and system parameters. Accordingly, the inventionis expressly not limited to those light division or split percentagesdisclosed herein which represent merely some of many possible designsthat might be used for the couplers. It will be appreciated by thoseskilled in the art that the determination of the optical split ratiodepends on how much light is intended to be directed into each of thesample and reference arms. It is desirable to have as much power aspossible on sample while keep the power on sample to be within a safelimit. In the meantime, sufficient power is needed on the reference armto get shot-noise limited sensitivity.

FIG. 5 is a 1951 USAF resolution test target image showing transverseresolution of the first prototype chip-based SDM-OCT system. The USAFtarget is widely accepted to test the resolution of optical imagingsystems such as microscopes, cameras and image scanners. Transverseresolution of the first prototype chip-based SDM-OCT system was measuredto be about 20 am with a USAF target (Group 4, element 5 is clearlyvisible). Note that the transverse resolution of the SDM-OCT systemdepends on the numerical aperture of the objective lens and can beincreased or decreased to fit the needs of different applications.

A method for processing light in a space division multiplexing opticalcoherence tomography system using integrated photonic chip 101 will bebriefly presented. In one embodiment, the method comprises: providing aphotonic chip comprising an optical input port, a plurality of opticaloutput ports, and a multiple branched waveguide structure opticallycoupling the input port to each of the output ports, the waveguidestructure comprising a plurality of interconnected waveguide channelsformed in the chip; receiving a singular sample beam from a light sourceat the input port; dividing the sample beam into a plurality of samplingbeams using a plurality of in-chip photonic splitters defined by thewaveguide channels in the splitter region; creating a time delay betweenthe plurality of sampling beams by varying a length of each waveguidechannel after dividing the sampling beam; and emitting the plurality ofsampling beams simultaneously in parallel through the output portstowards a sample to be scanned.

The method may further include steps of: receiving a plurality ofreflected light signals returned from the sample at the output ports;transmitting the reflected light signals through the time delay regionto the splitter region of the photonic chip; combining the reflectedlight signals into a singular reflected light signal; and emitting thesingular reflected light signal from the input port of the photonicchip. The method may further comprise steps of: creating an interferencesignal by combining the reflect light signal with a reference lightsignal; producing digital images of the sample using a digitizer basedon the interference signal. In one embodiment, the plurality of samplingbeams are transmitted through waveguide channels in the time delayregion of different lengths to create the time delay between theplurality of sampling beams.

Low Insertion Loss SDM-OCT System and Photonic Chip

Typically, when light is split from 1 fiber to N output channels using aphotonic chip 101 as described above, the intensity for each of theoutput fiber is about 1/N of the input intensity. This allows evendistribution of the light through all the output channels of thephotonic chip for sampling. When reflected light is collected andreturned from the sample passing back through the chip 101 again in thereverse direction, only about 1/N of the returned light is combined inthe input fiber 104 to the chip leading the reflected light back tocirculator 140. This insertion loss is proportional to how many channelsthe photonic chip 101 splits the light. In order to minimize theinsertion loss for the returned beam, an alternative embodiment of aphotonic chip-based SDM-OCT system 200 with chip 201 is presented inFIGS. 6 and 7.

SDM-OCT system 200 seeks to reduce return optical losses and couplinglosses at the input/output of the photonic chip 201. This design allowslight to be split only on the first pass through the photonic chip 101to the sample 191. Back-reflected light returned from the sample avoidsthe on-chip optical splitters and the input port 106 and associatedinput fiber 104 with this chip topology resulting in much lower losses.Unlike the photonic chip 101 of FIGS. 1-3 described above, reflectedlight signals returned from the sample 191 during the sampling processwhich produces the digitized images of the sample do not pass throughthe input fiber 104 of the present low loss photonic chip 201. The keydesign aspect in photonic chip 201, however, is that the returnedreflected light signals S2 will only pass through one row of couplers orsplitters 230 shown in the rectangular box in FIG. 7 indicating “samplesplitters”. The signal will be routed to interfere with reference light.This way, the top two rows of couplers or splitters 230 (each rowproduces 3 dB loss) from the input will be by-passed to avoid 6 dB lightloss. Therefore, it bears noting that bypassing the input port 206 andfiber 204 is a consequence of this bypass design implementation and isnot in itself the key factor.

Referring to FIG. 7, the low loss photonic chip 201 comprises asubstrate 202 which may have a generally rectangular prismatic or cuboidconfiguration in one embodiment comprising a parallel major surfaces 201a, 202 b defining a thickness T2 therebetween, and a four perpendicularside surfaces 201 c defining a perimeter of the chip. Substrate 202 isformed of a material having a first refractive index Ri1. In theprototype system, the substrate 202 may have a thickness T2 of about 1-2mm. Other thicknesses, however, may be used for substrate 202. Thesubstrate 202 may be made of any of the same materials suitable materialfor constructing a photonic chip as substrate 102 described above. Inone example without limitation, the substrate may be formed of silicon.

Low loss photonic chip 201 has a plurality of different input and outputports for receiving and transmitting light signals to and from the chipwhich travel or propagate through waveguide channels therein. In oneembodiment, chip 201 may have a sample beam input port 206 whichreceives an input sample light signal from the sample arm S sample beaminput optical fiber 204 generated by light source 110, a plurality ofoutput ports 205 which transmit sampling beams S1 to the sample 291 andreceive reflected light signals S2 returned from the sample, a referencesignal input port 207 receiving a reference signal R1 from the referencearm R, and a plurality of interference signal output ports 208 fortransmitting interference signals to digital balanced detector 280.Outputs ports 205 of the waveguide photonic chip 101 may be 8-degreeangle polished to reduce back reflections. Other polishing angles may beused control the amount of surface reflection.

Similarly to photonic chip 101, a multiple branched waveguide structurecomprising a plurality of waveguide channels 203 are formed in low losschip 201 and may lie in the same horizontal reference plane. Photonicchip 201 is patterned with an array or plurality of waveguide channels203 configured to form a splitter region, time delay region, andinterferometer region. Similarly to chip 101, the difference in therefractive indices at the interface of the waveguide channels 203 andbase substrate 202 confine and cause the light signal to travel withinthe waveguide channels 203 with minimal or no scattering of light intothe adjoining base portions of the substrate.

Referring to FIG. 7, the waveguide channels 203 may have the samecross-sectional geometry in some constructions. In one embodiment, thewaveguide channels 203 may be optimized for 1060 nm light operation.Ophthalmic swept-source OCT systems typically operate at 1060 nm centerwavelength to avoid the high absorption of water at the 1310 nmbandwidth. The cross sectional geometry of the waveguide channels 203must therefore be redesigned to ensure efficient propagation of lightsignals at the target wavelength. The waveguide channels may be designedfor other center wavelengths in other embodiments depending on theintended scanning application and requirements.

The waveguide channels 203 are patterned to form a three-row samplesplitter region similarly to chip 101 to split the incident singlesampling beam into eight output sampling beams or channels in thenon-limiting illustrated chip example. Other numbers of output samplingbeams may be used in other embodiments. The final or third row samplesplitters location is denoted in FIG. 7 by the first labelled dashed boxshown (the 1st and 2nd splitter rows are shown above the dashed box). Adistinct interferometer region is patterned on the chip 201 denoted bythe second labelled dashed box which receives 4 reference light signalsR1 each of which interferes with a reflected light signal S2 receivedfrom the sampling splitter region which collects the reflected lightreturned from the sample 291. The incident single reference light signalR1 is divided into the four reference light signals R1 by patterning thereflected light waveguide channels 203 with the appropriate number ofbranches as shown. In one embodiment, all the reference light R1waveguide channels may have the same optical path length whereas thesample arm sampling light S1 waveguide channels have different opticalpath lengths to produce the optical time delay.

In another embodiment, however, all the sampling light S1 waveguidechannels 203 may have the same optical path length while the referencelight R1 waveguide channels 203 instead have different optical pathlengths analogous to the above mentioned optical delays betweenchannels. In fact, a combination of sample arm and reference armwaveguide layout design may be used to generate the same differentialoptical path length delay between different interference signalsoriginated from different imaging channels. The optical path lengthdifference is used to shift the frequency of the interference signalfrom different imaging channels into different frequency bands, whichcorrespond to different depth ranges in the acquired OCT image.Accordingly, the invention is not limited to necessarily having the sameoptical path lengths for either the sample arm S or reference arm R. Theinterference signals from different channels are formed into differentfrequency bands when the optical path length difference betweenindividual sample arms and reference arms is unique. Since all theinterference signals are in different frequency bands, a singlephotodetector may be used to detect all the signals at oncesimultaneously in parallel.

FIG. 6 shows the SDM-OCT system 200 with the integrated low lossphotonic chip 201. Referring to FIGS. 6 and 7, optical coupling ofexternal devices of system 200 to the various waveguide channel ports ofthe photonic chip 201 may be made with optical fibers 204 in order toconnect to the rest of the SDM OCT system components shown. Light istransmitted from the long coherence swept source light source 110previously described herein through optical fiber 204 to a 20/80 opticalcoupler 210. Light source 110 in this embodiment may be a 200 kHz sweptsource laser to maximize the effective A scan rate while allowingsufficient dwell time for acceptable sensitivity. The coupler 210directs 20% of the light directed via an optical fiber to one of thethree ports in circulator 211 in the reference arm R. The remaining 80%of light emitted from coupler 210 is directed via an optical fiber tothe sampling arm S and photonic chip 201 to provide the sampling lightS1. Other splits for coupler 210 may be used in other embodiments asappropriate. The sampling light S1 is received at input port 206 of thechip.

The incident single beam sampling light S1 travels through the 3-rowcascade of photonic waveguide splitters 230 in chip 201 formed bywaveguide channels 203 which equally and gradually divide the incidentlight in each row into the final eight output sampling beams S1. Eachphotonic waveguide splitter 230 is formed by a branch in the waveguidewhich divides the input sampling beam S1 equally (i.e. 50/50) into twooutput sampling beams S1. This occurs in each of the three rows ofwaveguide splitters to gradually create 8 output sampling beams in theillustrated embodiment. Different splitter arrangements and/or number ofrows may be used in other embodiments as previously described herein. Atime delay is produced between each of the sampling light S1 beamsbetween the splitters 230 and output ports 205 in which the differenceΔL in the predetermined lengths between the waveguide channels isselected to produce an optical delay shorter than a coherence length ofthe light source between the plurality of sampling beams so that whenimages are formed, signals from different physical locations aredetected in different frequency bands. The 8-beam sampling light S1output from the chip 201 is then collimated by collimator 270 forfocusing on multiple discrete spots or sampling locations across thesurface of the sample 291. The plurality of sampling beams S1 aretransmitted to oscillating XY galvanometer mirror 173 to achievewide-field volumetric imaging of the sample 191 with SDM-OCT. A largescan lens 290 (objective lens) receives and irradiates the sample 291with the sampling beams S.

Reflected light signals S2 returned from sample 291 are captured byphotonic chip 201. Reflected sample light signals S2 travel back throughthe sample arm scan lens 290 and collimator 170 and are received by theoutput ports 205 on photonic chip 101. Light signals S2 each travelthrough one of the eight discrete waveguide channels 203 back to thelast third row of waveguide splitters 230. In the present embodiment,the waveguide splitters 230 in the final third horizontal row areconfigured with two inlets and two outlets instead of one inlet and twooutlets like the splitters 230 shown in the first and second rows of thesplitter region. A first inlet of splitters 230 in the third rowreceives sampling light S1 and splits the light into two sampling lightbeams transmitted through the two first and second outlets. The secondinlet forms a reflected sample light S2 output which directs thereflected light signals returned from sample 291 to the interferometerunit or region of photonic chip 201, which in turn comprises a pluralityof waveguide photonic interferometers 231 formed by waveguide couplersor splitters 230 also having four branches. Each third row waveguidesplitter 230 combines two of the eight reflected light signals S2 toform four reflected light signals as shown.

Each waveguide photonic interferometer 231 receives one of fourreflected sample light S2 signals transmitted by the foregoing third rowwaveguide splitters 230 and one of four corresponding reference lightsignals R1 formed by waveguide splitters 230 which divide the single thereference light signal received from the reference arm R by the photonicchip 201 into four via a two row cascade of splitters.

Reflected light signals S2 returned from sample 291 and reference lightR1 from reference arm R are combined to create a plurality ofinterference signals I. The interference signal I generated by eachwaveguide photonic interferometer 231 are split into two interferencesignals I as shown which are transmitted to the interference signaloutput ports 208 on chip 201. The output ports 208 are grouped into twofiber bundles FB1 and FB2 each comprising a plurality of optical fiber204; one fiber coupled to each one of the output ports 208. The fibers204 transmits interference signals I to a single balanced detector 280.The output signal from detector 280 is acquired by a dual-channelhigh-speed data acquisition card 183. The detected signal will bedigitized with the high speed data acquisition card 183 and streamed tocomputer 184 to generate OCT scan images of the sample captured bySDM-OCT system 200.

In the foregoing scheme, the desired optical delay between each samplinglight S1 can be adjusted by varying the length of each waveguide channel203 as needed in the time delay region 109 of the photonic chip 201 tocreate a delay between each channel in the same manner previouslydescribed herein. The temporal delay is a function of the optical pathlength of each of the channels. In this embodiment, one skilled in theart can optimize the optical or time delay for use with a 200 kHz sweptsource laser to maximize the effective A scan rate while allowingsufficient dwell time for acceptable sensitivity. The spacing betweenthe output sampling light S1 beams can also be independently adjusted toproject the beams on the sample with a specific physical separation onthe surface of the sample between imaging regions.

The waveguide channels 203 and structures such as the cascading samplebeam splitters and interferometers shown in FIG. 7 may be formed in thesame horizontal plane on photonic chip 201 in one embodiment. Althoughsome portions of the waveguide channels 203 intersect or cross eachother as shown, cross-talk between the waveguides at the intersectionswill be very small (−40 dB) with proper design in accordance withmethods known in the art (see, e.g. Y. Ma et al., “Ultralow loss singlelayer submicron silicon waveguide crossing for SOI opticalinterconnect,” Optics Express 21(24), 29374-29382 (2013)). In otherembodiments, the waveguide channel optical structures in photonic chip201 may be arranged in a three dimensional manner such that differentoptical structures may be found in two or three vertical layers orlevels within the chip using semiconductor fabrication techniques knownin the art and used for building multi-level circuits. Such a chiparchitecture could completely or substantially avoid crosses orintersections between the waveguides to virtually eliminate cross talkand provide even better performance.

FIG. 8 illustrates how all of the traditional optical coupler/splitterdevices can advantageously be replaced by the single low loss photonicchip 201 with the foregoing on-chip patterned photonic components andwaveguide structures shown in FIG. 7. All of the traditional opticaldevices shown within the dashed box are replaced by the photonic chip201.

FIG. 9 illustrates another embodiment of the low loss photonic chip 201.The layout of the chip is similar to FIG. 7, except that output fibers(FB1s and FB2s) are completely eliminated. Instead, outputs from theinterferometers 231 are grouped and coupled directly into the + and −photodetectors 304, 306 (PD+ and PD−) used for the balanced detector280. The PD+ and PD− may be directly attached such as epoxied on theside of the photonic chip to integrate the photonic chip with thedetection electronics. The advantage of this embodiment is that thecoupling and transmission loss of the fiber bundles are eliminated.

In addition, FIG. 10 illustrates an embodiment of the low loss photonicchip, where photodetectors 304, 306 (PD+ and PD−) and detectionelectronics may be directly integrated on the chip as active components.In fact, the silicon substrate is often used to develop electro-opticaldevices, including photodetectors, phase modulators, and polarizationcontrollers, etc. Such techniques are described for example in A. Cutoloet al.; Silicon electro-optic modulator based on a three terminal deviceintegrated in a low-loss single-mode SOI waveguide, Journal of LightwaveTechnology, 15(3), 505-518, 1997, and other technical publications.Phase modulators 302 and polarization controllers 300 may thus beincluded in each of the sampling channels as shown in the embodiments ofboth FIGS. 9 and 10. This allows individual control of the phase andpolarization states of the light incident on the sample and returned onthe interferometers in order to achieve optimal OCT image quality.

A method for processing light in a space division multiplexing opticalcoherence tomography system using low loss integrated photonic chip 201will now be briefly described. In one embodiment, the method comprises:providing a photonic chip comprising an optical input port, a pluralityof optical output ports, and a multiple branched waveguide structureoptically coupling the input port to each of the output ports, thewaveguide structure comprising a plurality of interconnected waveguidechannels formed in the chip; receiving a singular sample beam from alight source at the input port; dividing the sample beam into aplurality of sampling beams using a plurality of in-chip photonicsplitters defined by the waveguide channels in the splitter region;creating a time delay between the plurality of sampling beams by varyinga length of each waveguide channel after dividing the sampling beam; andemitting the plurality of sampling beams simultaneously in parallelthrough the output ports towards a sample to be scanned; receiving aplurality of reflected light signals returned from the sample at theoutput ports; transmitting the reflected light signals to a plurality ofinterferometers defined by the waveguide channels in an interferometerregion of the photonic chip; combining the reflected light signals witha reference light signal using the plurality of interferometers togenerate a plurality of interference signals; and emitting theinterference signals from interference output ports of the photonicchip.

It expressly bears noting that although somewhat distinct on-chipphotonic beam splitter and optical time delay regions are shown in theillustrated embodiments of photonic chips 101 and 201 for simplicity ofdesign and fabrication, the present invention does not necessarilyrequire that the beam splitting and time delay functionality beperformed in separate regions or areas of the chips. Accordingly, thebeam splitting and time delay functions may be performed in a singlecombined or mixed splitter and time delay region or area of the chips inother embodiments contemplated in which optical light path lengths. Forexample, different optical path or waveguide channel lengths may be usedbetween successive rows of splitters to create the optical delay.

It expressly bears noting that although several linear rows of in-chipphotonic splitters 107, 230 are disclosed herein, other embodiments mayhave more random or staggered arrangements of splitters which are notlinear and/or may not be aligned into rows. Accordingly, the inventionis not limited to formation of cascading linear rows of splittersrepresented merely one of many possible configurations and arrangementsof splitters that may be formed by the waveguide channels 103 and 203.

While the foregoing description and drawings represent exemplaryembodiments of the present invention, it will be understood that variousadditions, modifications and substitutions may be made therein withoutdeparting from the spirit and scope and range of equivalents of theaccompanying claims. In particular, it will be clear to those skilled inthe art that the present invention may be embodied in other forms,structures, arrangements, proportions, sizes, and with other elements,materials, and components, without departing from the spirit oressential characteristics thereof. In addition, numerous variations inthe methods/processes as applicable described herein may be made withoutdeparting from the spirit of the invention. One skilled in the art willfurther appreciate that the invention may be used with manymodifications of structure, arrangement, proportions, sizes, materials,and components and otherwise, used in the practice of the invention,which are particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being defined by the appended claims andequivalents thereof, and not limited to the foregoing description orembodiments. Rather, the appended claims should be construed broadly, toinclude other variants and embodiments of the invention, which may bemade by those skilled in the art without departing from the scope andrange of equivalents of the invention.

What is claimed is:
 1. An integrated photonic chip suitable forspace-division multiplexing optical coherence tomography scanning, thephotonic chip comprising: a substrate; an optical input port configuredto receive an incident singular sampling beam from an external lightsource; a plurality of optical output ports configured to transmit aplurality of sampling beams from the chip to a sample to capture scannedimages of the sample; and a multiple branched waveguide structureoptically coupling the input port to each of the output ports, thewaveguide structure comprising a plurality of interconnected waveguidechannels formed in the substrate; the waveguide channels configured todefine a plurality of photonic splitters which divide the incidentsingular sampling beam received at the input port into the plurality ofsampling beams at the output ports; wherein portions of the waveguidechannels between the photonic splitters and output ports have differentpredetermined lengths to create an optical time delay between each ofthe plurality of sampling beams; wherein a difference in thepredetermined lengths between the waveguide channels is selected toproduce an optical delay shorter than a coherence length of the lightsource between the plurality of sampling beams so that when images areformed, signals from different physical locations are detected indifferent frequency bands.
 2. The photonic chip according to claim 1,wherein the photonic splitters are arranged in multiple cascading rowson the substrate, the singular sampling beam being successively dividedin each row by the photonic splitters to create an increasingly greaternumber of sampling beams in each row between the inlet port and theoutput ports.
 3. The photonic chip according to claim 1, wherein theoutput ports emit the sampling beams from the photonic chip directlyinto air to the sample.
 4. The photonic chip according to claim 1,wherein the output ports are arranged to receive a plurality ofreflected light signals returned from the sample, the photonic splittersbeing configured to combine the plurality of reflected light signalsinto a singular reflected light signal which is emitted from the inputport of the photonic chip.
 5. The photonic chip according to claim 1,wherein the plurality of output ports are clustered together on one sideof the substrate and evenly spaced apart at a predetermined pitchspacing.
 6. The photonic chip according to claim 5, wherein a differencein length between each adjacent waveguide channel in the photonic chipis the same.
 7. The photonic chip according to claim 1, furthercomprising an optical fiber coupled to the input port of the photonicchip.
 8. The photonic chip according to claim 1, wherein the substrateis selected from the group consisting of silicon, silicon on insulator,Iridium Phosphide, Lithium Niobate, Silicon Nitride and GalliumArsenide.
 9. The photonic chip according to claim 1, wherein thesampling beams in the time delay region travel in a path generallyperpendicular to a path of the sampling beams in the splitter region.10. The photonic chip according to claim 1, wherein the waveguidechannels are etched into the substrate.
 11. A low loss integratedphotonic chip suitable for space-division multiplexing optical coherencetomography scanning, the photonic chip comprising: a substrate; anoptical input port configured to receive an incident singular samplingbeam from an external light source; a reference light input portconfigured to receive reference light from an external reference lightsource; a plurality of optical output ports configured to transmit aplurality of sampling beams from the chip to a sample to capture scannedimages of the sample; a multiple branched waveguide structure opticallycoupling the input port to each of the output ports, the waveguidestructure comprising a plurality of interconnected waveguide channelsformed in the substrate, the waveguide channels defining a splitterregion and an interferometer region; the waveguide channels in thesplitter region configured to define a plurality of photonic splitterswhich divide the incident singular sampling beam received at the inputport into the plurality of sampling beams at the output ports; whereinportions of the waveguide channels between the photonic splitters andoutput ports have different predetermined lengths to create an opticaltime delay between each of the plurality of sampling beams; thewaveguide channels in the interferometer region configured to define aplurality of photonic interferometers, the photonic interferometersoptically coupled to the waveguide channels in the time delay region andthe reference light; wherein the photonic interferometers are arrangedto receive a plurality of reflected light signals returned from thesample, the photonic interferometers being configured and operable tocombine the reflected light signals with the reference light to producea plurality of interference signals which are emitted from interferencesignal output ports of the photonic chip.
 12. The photonic chipaccording to claim 11, wherein the photonic splitters are arranged inmultiple cascading rows on the substrate, the singular sampling beambeing evenly and successively divided in each row by the photonicsplitters to create an increasingly greater number of sampling beams ineach row between the inlet port and the output ports.
 13. The photonicchip according to claims 12, wherein the photonic interferometers areoptically coupled to photonic splitters in a final row of the splitterregion.
 14. The photonic chip according to claim 13, wherein thereflected light signals returned from the sample travel through thephotonic splitters in the final row to the interferometers and bypasspreceding rows of photonic splitters in the splitter region.
 15. Thephotonic chip according to claim 11, wherein the photonicinterferometers are optically coupled to the reference light input portvia a plurality of reference light waveguide channels.
 16. The photonicchip according to claim 11, wherein a difference in the predeterminedlengths between the waveguide channels is selected to produce an opticaldelay shorter than a coherence length of the light source between theplurality of sampling beams so that when images are formed, signals fromdifferent physical locations are detected in different frequency bands.17. A method for processing light in a space division multiplexingoptical coherence tomography system using a low loss integrated photonicchip, the method comprising: providing a photonic chip comprising anoptical input port, a plurality of optical output ports, and a multiplebranched waveguide structure optically coupling the input port to eachof the output ports, the waveguide structure comprising a plurality ofinterconnected waveguide channels formed in the chip; receiving asingular sample beam from a light source at the input port; dividing thesample beam into a plurality of sampling beams using a plurality ofin-chip photonic splitters defined by the waveguide channels in thesplitter region; creating a time delay between the plurality of samplingbeams by varying a length of each waveguide channel after dividing thesampling beam; and emitting the plurality of sampling beamssimultaneously in parallel through the output ports towards a sample tobe scanned; receiving a plurality of reflected light signals returnedfrom the sample at the output ports; transmitting the reflected lightsignals to a plurality of interferometers defined by the waveguidechannels in an interferometer region of the photonic chip; combining thereflected light signals with a reference light signal using theplurality of interferometers to generate a plurality of interferencesignals; and emitting the interference signals from interference outputports of the photonic chip.